1. Field of the Invention
The present invention generally relates to apparatus and method for particle sizing of the type which employs laser diffraction to measure particle size. The present invention, more specifically, uses, with other components, a monomode optical fiber for producing a beam of light having a high degree of spatial coherence in a spatial filter that is easily aligned, replaceable, rugged and cost effective.
2. Description of Prior Art
The use of laser light diffraction to measure particle size is a widely known technique. Laser diffraction is a particle sizing method which uses the average relative angular intensity of scattered light. Instruments that use laser light diffraction to measure particle size have been available for many years from a number of different manufacturers. All laser diffraction instruments use the same basic method to measure particle size. All laser diffraction instruments require a beam of monochromatic light with a very uniform wave front. This beam of laser light is directed at the sample particles to be measured. When the light hits the particles, the light is diffracted or scattered from the particles. Detectors are used to measure the relative average intensity of the light scattered at various angles from the sample material. Once the relative intensity of light scattered at several different angles from the particles is known, the particle size and size distribution can be calculated.
The ability to make accurate measurements of particle size is directly related to the quality of the beam, its spatial coherence, which illuminates the sample particles. This monochromatic light beam must be highly collimated, meaning that all the rays of light traveling in the beam are parallel to one another.
In order for a beam of light to be highly collimated, the light must have a very uniform wave front from the light source. Ideally the light source would be a perfect point source of light, infinitesimally small. Also, the light source must be free of diffraction, which could be caused by dust particles in the air, or because the light beam is partially obstructed. In addition, any optical lenses used to collimate the beam of light must be free of surface and material imperfections which would also cause light diffraction. Finally, any optical lenses use to collimate the beam must be designed to minimize any aberrations caused by the lens itself. These characteristics are necessary to achieve high resolution size measurements.
Apparatus and method using laser diffraction to measure particles is importantly different from dynamic light scatter apparatus and method for particle analysis. Dynamic light scatter requires time fluctuation, or power spectral measurement of the scattered light. Whereas, laser diffraction requires measurement of average, relative angular intensity of the scattered light at a number of detection angles, which is not a time or frequency based measurement. The basic differences between structures, methods, and optical requirements of laser diffraction versus dynamic light scatter are known to those in these fields. However, some sophisticated and subtle differences of laser diffraction might not be appreciated by those knowledgeable in dynamic light scatter technology.
In laser diffraction devices, such as the COULTER.RTM. LS and competitive devices, spatial filtering of the laser beam is used to create the above discussed high spatial coherence quality beam and is one of the most important aspects of the instrument. In the COULTER LS, in order to measure the small angular deflection of the laser beam caused by diffraction from very large particles such as nine hundred micrometers (900 .mu.m), light scattered at angles as small as 0.5 milliradians (mR) must be measured and an angular resolution of approximately 0.05 mR is desired. To achieve this level of beam quality, the laser beam must be expanded to about thirteen millimeters (13 mm) and a diffraction limited beam of this diameter must be formed by the collimating optics. A diffraction limited Gaussian beam of thirteen (13) mm diameter, with a wavelength of seven hundred and fifty nanometers (750 nm), has a divergence of .about.0.04 mR. Any serious discrepancy between this desired level of spatial coherence and collimation and the actual performance leads to degradation in the resolution of the instrument.
All particle sizing instruments based on laser diffraction techniques use a spatial filter to provide this very high quality beam of laser light. All of these spatial filters use a pinhole in combination with other optical elements to create the required quality light beam. A pinhole is a small, circular hole in a thin, flat piece of rigid, opaque material. A typical pinhole spatial filter is configured in the following manner. A source of light, such as a laser diode, illuminates a circular beam stop, which makes the light beam circular. The circular light beam then passes through a system of optical lenses. These lenses focus the laser beam down to the pinhole, which is typically between twenty to fifty (20-50) .mu.m in diameter, allowing most of the light beam to pass through the pinhole. Any impurities in the laser light, caused by diffraction or lens aberration do not pass through the pinhole, but are blocked by the opaque material surrounding the pinhole. The light that passes through the pinhole is then "clean," except for some diffraction rings caused by the pinhole itself. These diffraction rings are removed by another beam stop placed at the exact minimum of the first diffraction ring. Finally, a lens collimates this diverging, circular beam of light at the point the desired beam diameter, thirteen (13) mm in the case of the COULTER LS, is reached, creating a highly collimated, uniform wave front beam of light, which is useful for laser diffraction particle sizing.
While the pinhole method of creating this beam of light works effectively, in practice it has many problems. First, in order to pass most of the light from the laser source through the pinhole, the optical elements including the source, the first beam stop, the lenses and the pinhole, must be precisely focused and aligned to within a few micrometers. This requires the use of very complicated and expensive mechanical elements to provide the fine resolution these adjustments require. Additionally, the time required to sufficiently adjust the assembly can be many hours. Secondly, once the assembly is fully aligned, it can be easily misaligned by mechanical distortions from clamping, or from temperature changes, which cause the various components to expand per their respective coefficients of thermal expansion. Also, shock and vibration during shipment of the instrument can cause the pinhole assembly to become misaligned, causing expensive, time consuming field service. Once in use in the laboratory, if a component of the spatial filter optical train burns out or is damaged, the entire optical assembly must be returned to the factory for parts replacement and then the time consuming, expensive optical realignment.
Thus, it would be advantageous for laser diffraction particle analysis apparatus to improve upon the pinhole style of spatial filter assembly to reduce or eliminate the above mentioned drawbacks. Alternatively, if the pinhole and other associated components could be replaced to provide an assembly that is much more rugged, much more immune to distortions caused by thermal effects, shock and vibration, requires very little alignment, and is lower cost, such replacement would solve a longstanding need.
Many devices, for example those described in one or more of the hereinafter listed publications, utilize various forms of optical fibers, including monomode and multimode fibers, in light transmitting and light detecting arrangements. However, none of the prior art devices describe a monomode optical fiber apparatus in a spatial filter capable of providing the high quality light beam required for particle sizing using laser diffraction techniques.
U.S. Pat. No. 4,953,978, Steven E. Bott et al., Coulter Electronics of New England, Inc., PARTICLE SIZE ANALYSIS UTILIZING POLARIZATION INTENSITY DIFFERENTIAL SCATTERING. PA0 U.S. Pat. No. 4,975,237, Robert G. W. Brown, The Secretary of State for Defence in Her Britannic Majesty's Government of the United Kingdom of Great Britain and Northern Ireland, DYNAMIC LIGHT SCATTERING APPARATUS. PA0 U.S. Pat. No. 5,056,918, Steven E. Bott et al., Coulter Electronics of New England, Inc., METHOD AND APPARATUS FOR PARTICLE SIZE ANALYSIS. PA0 Juskaitis, R., et al., 1992, Electronics Letters Vol. 28(11), FIBRE-OPTIC BASED CONFOCAL SCANNING MICROSCOPY WITH SEMICONDUCTOR LASER EXCITATION AND DETECTION. PA0 Brown, R. G. W., et al., 1987, J. Physics E, Vol. 20, MONOMODE FIBRE COMPONENTS FOR DYNAMIC LIGHT SCATTERING. PA0 Brown, R. G. W., 1988, HMSO, MINIATURE INSTRUMENTATION FOR LASER LIGHT SCATTERING EXPERIMENTS. PA0 Knuhtsen, J., et al., 1982, The Institute of Physics, FIBRE-OPTIC LASER DOPPLER ANEMOMETER WITH BRAGG FREQUENCY SHIFT UTILISING POLARISATION-PRESERVING SINGLE-MODE FIBRE. PA0 Brown, R. G. W., 1987, Applied Optics, Vol. 26(22), DYNAMIC LIGHT SCATTERING USING MONOMODE OPTICAL FIBERS. PA0 Dabbs, T., et al., 1992, Applied Optics, Vol. 31(16), FIBER-OPTIC CONFOCAL MICROSCOPE: FOCON. PA0 Brown, R. G. W., 1987, DESIGNS OF FIBRE OPTIC PROBES FOR LASER ANEMOMETRY: Paper 9, Second International Conference on Laser Anemometry--Advances and Applications, Strathclyde, UK.
U.S. Pat. No. 4,975,237 to Brown relates to the use of monomode optical fibers in a light detector assembly, in a dynamic light scatter apparatus. Brown describes the substitution of a pinhole in front of a photo detector with a monomode optical fiber, the purpose of which is to isolate a small area of light from a large amount of scattered light from the particles. Brown uses a monomode optical fiber simply because the core diameter of the monomode fiber is of approximately the correct size to isolate a single coherence area of scattered light. Brown does not use an optical fiber as a light delivery and filtering device suitable for laser diffraction. In FIG. 1 of Brown, a monomode optical fiber is shown in a beam delivery path. Brown does not, however, teach or suggest benefits of spatial filtering employing the monomode optical fiber, because the Dynamic Light Scattering method of his device does not require the beam quality required of the laser diffraction sizing apparatus.
Other apparatus, such as the confocal microscope of Juskaitis et al., use monomode optical fibers for both delivery and detection of light. Such devices and their methods are not related to laser diffraction particle sizing and do not teach the use of monomode optical fibers in spatial filter assemblies for such.